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HYDROLOGY AND WATER QUALITY Environmental Setting Climate The northern San Joaquin Valley has a warm, two season climate with cool wet winters and hot dry summers. In Stockton, an average of 14 inches of rain falls yearly, with 90 percent of the rainfall occurring between November and April . Rainfall in the Coast Range to the west is generally greater than that on the valley floor, averaging approximately 20 inches per year. The Sierra Nevada Range to the east has a four season climate with short mild summers and long cold winters. Annual precipitation in the northern Sierras averages from approximately 30 inches in the foothills to as high as 70 inches in the higher elevations. About 90 percent of the annual precipitation falls from November to April , primarily as snow (Department of Water Resources 1960) . Topography and Drainage The project site is located on the eastern fringe of the San Joaquin River Delta, approximately seven miles east of the main channel . The southern Delta area is the ultimate destination for the entire system of drainages in the San Joaquin Valley, which includes most of the west slope of the Sierra Nevada and eastern slope of the southern Coast Range. Excluding the Tulare Lake basin, San Joaquin County is the terminus for over 9,000 square miles of drainage area (San Joaquin County Department of Planning and Building Inspection 1978) . Major tributaries of the San Joaquin River upstream from the project area include the Stanislaus, Tuolomne and Merced Rivers which drain the west slope of the Sierra east of Lathrop. The Stanislaus River flows into the San Joaquin River approximately 16 miles southwest of the project area. The topography of the project site is relatively flat with an aver- age elevation of 45 feet and water flowing from east to west. Drainage is collected in a temporary ditch running along Austin Road which forms 3-38 101-45.R4 4/10/89